I have an old 2008 MacBook with 4GB of memory. (Memory from About this Mac, everymac.com gave me a different number.) I know it's old, but it almost seems like it's slower than it should be. I have a lot of software so I was wondering, how could I easily find memory leaking processes?
Activity Monitor.app indicates how much memory each process running on your Mac is using.
To find apps which are using the most memory, try the following command in Terminal:
ps -awxm -o %mem,rss,comm | sort -nr | head
top command may help to monitor suspicious processes in real time.
In general OS X kernel (similar to others) is designed to use up all the available memory. See: Isn't Inactive memory a waste of resources? for more details.
If you've limited amount of memory, consider quitting apps or closing browser tabs which you're not using. You can also find some Memory Clean apps which will try free up some memory when it's full.